This invention relates to a novel syntactic foam composition and to a method for insulating subsea apparatus such as oil pipelines with the novel syntactic foam composition. More particularly, this invention relates to a novel syntactic foam composition having density, strength and thermal conductivity characteristics which permit its use as an insulation material for subsea apparatus.
Subsea oil pipelines deliver oil from subsea wellheads to surface devices for further distribution. The oil emitted from subsea wells is hot and can endure only a certain amount of cooling during transfer through a pipeline to the surface through ambient seawater without thickening or evolving ices or clathrates which can plug pipelines and reduce the volume flow rate of oil to the surface. The subsea wellhead can be located as deep as 4000-5000 feet or more below the sea surface. To insulate the oil during this transit to the surface, thus keeping it low in viscosity and free-flowing, syntactic insulation is applied to the pipeline by several methods. One of the methods is a pre-cast form which also generally provides buoyancy to the pipeline to stabilize and reduce the weight of the pipeline applied at the surface.
Syntactic insulation refers to a particular type of foam where the foam gas inclusions are provided by means of hollow spherical particles. These hollow spherical particles encapsulate the insulating gas protecting the insulation from collapse due to the high pressures encountered in the subsea environment.
The depth of exposure in the seawater determines the required strength of the syntactic foam in the composite. The strength of the composite is related to the strength of the ingredients, especially the hollow spheres, which primarily determine the low density. The insulating ability of the syntactic foam also is determined by the overall density of the composite. The lower the density, the better the insulating ability since heat travels via transfer of molecular motion from molecule to molecule. The rate of this transfer is a function of the material of transfer and is most directly dependent on density. A vacuum is the best thermal insulator and low density gases such as those filling hollow spheres are superior in general to liquids and solids including composite binders such as epoxies, fillers such as fibers or even the solid walls of hollow spheres made, for example, from glass or epoxy/fiber.
Thus, both buoyancy and insulative capacity are a function of composition, density, and volume filling of hollow spheres. The volume filling of hollow spheres in a syntactic composite article is a function of hollow sphere densities and packing factor(s) of the hollow spheres. The packing factor is related to size and size distribution and is measured as the ratio of bulk density to true particle density. Maximum packing and minimum density is obtained in a composite foam when spheres are touching. Practically, this extreme is realized by a sharp rise in viscosity of the resin/hollow sphere mix prior to any curing. At this point, any further addition of hollow spheres results in a dry mix, incapable of further processing without additional liquid addition.
Packing factors can be improved by using two distinct hollow sphere types in combination which differ in size by, at least, a factor of seven. If a hollow sphere with a size of 100 microns is combined with a hollow sphere with a size of less than 15 microns, or conversely with a hollow sphere greater than 700 microns in size, increased packing is possible because the smaller sphere fits in the interstices between the larger spheres. Resin binder, such as epoxy, then fills the rest of the volume.
It has been proposed in U.S. Pat. No. 3,622,437 to provide syntactic foam compositions having a specific gravity less than one from a composition comprising a resin binder, macrospheres having a diameter of one-half to four inches and smaller spheres having a diameter of one-seventh or less than the macrospheres. While the foam compositions provide sufficient strength and buoyancy to permit their use as pipeline insulators in limited environments, they are not capable of providing the necessary combination of strength, buoyancy and low thermal conductivity at subsea service depths up to 10,000 fee at reasonable insulation thicknesses so that constant flow of petroleum can be maintained through the pipeline. This deficiency is particularly evident when the foam composition is positioned within a carrier pipe of fixed diameter which surrounds the pipeline for the petroleum.
Accordingly, it would be desirable to provide syntactic foam composition which has the buoyancy, strength and thermal insulating properties which permit its practical use as an insulating composition for subsea apparatus such as oil pipelines.